src: give StreamBases the capability to ask for data

Add a `OnStreamWantsWrite()` event that allows streams to
ask for more input data if they want some.

PR-URL: https://github.com/nodejs/node/pull/18936
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

src/node_http2.cc

@@ -2219,6 +2219,11 @@ ssize_t Http2Stream::Provider::Stream::OnRead(nghttp2_session* handle,
   if (amount == 0 && stream->IsWritable()) {
     CHECK(stream->queue_.empty());
     DEBUG_HTTP2SESSION2(session, "deferring stream %d", id);
+    stream->EmitWantsWrite(length);
+    if (stream->available_outbound_length_ > 0 || !stream->IsWritable()) {
+      // EmitWantsWrite() did something interesting synchronously, restart:
+      return OnRead(handle, id, buf, length, flags, source, user_data);
+    }
     return NGHTTP2_ERR_DEFERRED;
   }
 

src/node_http2.h

@@ -573,6 +573,8 @@ class Http2Stream : public AsyncWrap,
   // Required for StreamBase
   int DoShutdown(ShutdownWrap* req_wrap) override;
 
+  bool HasWantsWrite() const override { return true; }
+
   // Initiate a response on this stream.
   inline int SubmitResponse(nghttp2_nv* nva,
                             size_t len,

src/stream_base-inl.h

@@ -136,6 +136,13 @@ inline void StreamResource::EmitAfterShutdown(ShutdownWrap* w, int status) {
   listener_->OnStreamAfterShutdown(w, status);
 }
 
+inline void StreamResource::EmitWantsWrite(size_t suggested_size) {
+#ifdef DEBUG
+  v8::SealHandleScope handle_scope(v8::Isolate::GetCurrent());
+#endif
+  listener_->OnStreamWantsWrite(suggested_size);
+}
+
 inline StreamBase::StreamBase(Environment* env) : env_(env) {
   PushStreamListener(&default_listener_);
 }

src/stream_base.h

@@ -131,6 +131,13 @@ class StreamListener {
   // (and raises an assertion if there is none).
   virtual void OnStreamAfterShutdown(ShutdownWrap* w, int status);
 
+  // This is called by the stream if it determines that it wants more data
+  // to be written to it. Not all streams support this.
+  // This callback will not be called as long as there are active writes.
+  // It is not supported by all streams; `stream->HasWantsWrite()` returns
+  // true if it is supported by a stream.
+  virtual void OnStreamWantsWrite(size_t suggested_size) {}
+
   // This is called immediately before the stream is destroyed.
   virtual void OnStreamDestroy() {}
 
@@ -199,6 +206,9 @@ class StreamResource {
                       size_t count,
                       uv_stream_t* send_handle) = 0;
 
+  // Returns true if the stream supports the `OnStreamWantsWrite()` interface.
+  virtual bool HasWantsWrite() const { return false; }
+
   // Optionally, this may provide an error message to be used for
   // failing writes.
   virtual const char* Error() const;
@@ -222,6 +232,8 @@ class StreamResource {
   void EmitAfterWrite(WriteWrap* w, int status);
   // Call the current listener's OnStreamAfterShutdown() method.
   void EmitAfterShutdown(ShutdownWrap* w, int status);
+  // Call the current listener's OnStreamWantsWrite() method.
+  void EmitWantsWrite(size_t suggested_size);
 
   StreamListener* listener_ = nullptr;
   uint64_t bytes_read_ = 0;